1. Field of the Invention
This invention relates to drilling of wells, and more particularly to stabilization of boreholes drilled through subterranean formations including at least one unconsolidated sandstone zone and at least one shale zone, such as in drilling for oil and/or natural gas.
A problem often encountered in drilling through a shale zone is that the drilling fluid causes the shale to swell and slough off, resulting in a non-uniform borehole.
A problem often encountered in drilling through unconsolidated sandstone zones, and especially potential pay zones, is that of degradation of the borehole wall in the completion zone.
2. The Prior Art
A shale-stabilizing drilling fluid additive comprised of certain high molecular weight polymers and potassium salts is described in European Patent application No. 89302558.5. This additive when used in water-based drilling fluid effectively stabilizes the shale so that the borehole does not lose physical and dimensional integrity when exposed to water based drilling fluids.
U.S. Pat. No. 4,957,174 describes a process of correcting lost circulation of drilling fluid by adding particulate coke particles to a drilling fluid. This is a remedial process not related to formation stability.
In pending and commonly assigned U.S. application Ser. No. 07/785,647, now U.S. Pat. No. 5,018,020, a process is described in which formation fracture and propagation during drilling are prevented or inhibited by addition of finely divided (250-600 microns) particulate material to the drilling fluid.